Natural gas and water coexist in the stratum at where a gas production well is drilled. Since the specific gravities of a natural gas is lower than that of water, the natural gas is stored over the water. During the process of the natural gas production, the pressure within the system would gradually drop, and formation of water would gradually rise to approach the production well being drawn and tend to enter the production well to adversely affect the gas production.
Generally, entering of water into a production well might be caused by different reasons, such as a high permeability sub-formation at the production zone that results in an earlier edge encroachment on the well; vertical cracks or faults within the production zone in resulting the water that is formed in the bottom flowing into the upper production zone; fissures on a casing or a plug set in the well; and poor cement bond at the casing of the well. A rising water gas contact (WGC) might even induce water coning to result in an increased water flowing rate and a shutoff of the well. Under these conditions, the water must be completely shut off lest it should rise and invade the well to adversely affect the gas production at other zones.
In the case of a gravel packed production well subjected to water invasion, since the gravels between a screen pipe and a wall of the well is highly permeable and there is not a casing provided at the production zone, both of the mechanical type bridge plug and the thermal expansion type patch flex are not applicable to block the perforated zone under the WGC for the purpose of continuing the gas production of the well.
The conventional bridge plug can only be mounted in the screen pipe, and does not provide the function of blocking tiny meshes on the screen pipe and the gravels outside the screen pipe. That is, the conventional bridge plug does not function to seize information water from flowing into the well. Similarly, the thermal expansion type patch flex must also be provided in the screen pipe. As to the conventional way of pumping gel into the gravel packed production well using coiled tubing, since the tiny meshes on the screen pipe and the gravels outside the screen pipe are highly permeable, the pumped gel would inevitably flow through the gravels and fail to completely enter a water-out zone in the production well.
Regarding the way of placing traditional G-grade or H-grade cement milk at the bottom of the well to block the water-out zone, it is also not applicable because the cement has large particle size and cannot pass through the screen pipe into the gravels easily. Therefore, once the bottom of the gravel packed production well is subjected to invasion by water, there is no way to save the well but leaving it to the rising water until the well is no longer economical for use and shut off, or until the invaded water accumulates in the oil pipe and the well no longer produces any oil or gas.
For gravel packed production wells, a most common way of stopping water invasion is to use thermosetting resin or fine-grained cement milk to block the water-out zone in the production well. In most cases, the resin used for this purpose is phenolic resin; however, in the case of stopping water invasion with the thermosetting resin, the formation water would still break through a joint of the gravels and the well wall to largely reduce the water blocking effect, provided the differential pressure at the well bottom exceeds 50 pounds when the well resumes production. There is also a limitation to the cement used to block water. Only the ultra-fine-grained cement can flow into the gravels to completely fill up the pore space thereof, and thus blocks the water-out zone after the cement is cured.
In performing water blocking for the gravel packed production well, a dump bailer is used to carry the cement milk, resin, or other types of treating fluids. The dump bailer is a cylindrical container being lowered to the bottom of the well or a desired depth in the well using a wire line. A signal is transmitted from the ground to open the dump bailer and release the treating fluid (i.e., resin or cement milk) loaded therein. Due to a difference in specific gravity between the treating fluid and the formation water, the treating fluid is able to flow into the gravels between the screen pipe and the well wall. When the treating fluid is cured, the water-out zone is blocked.
The conventional water blocking method as described above has several disadvantages:
(1) The cement milk or resin mixed with the formation water is diluted to adversely affect the subsequent curing of the cement milk or resin.
(2) The gravel layer must not be overly contaminated to result in uneven permeability thereof. The gravel layer is used to retard sand. When the well has produced gas for a period of time, the gravel layer thereof would inevitably be contaminated by dust and sand from the producing formation. Under this condition, the cement milk or resin that flowed into the gravel layer are simply due to the gravity force not being able to fully fill all the pores in the gravel layer and completely block the water coning at the bottom of the well.
(3) When a production well is subjected to water invasion, a water column is accumulated at the bottom of the well. When the cement milk or the resin is dumped into the bottom of the well, the formation water would form a thin membrane on the well wall. When the cement milk or the resin is cured and the well is reopened for production, a large pressure difference as high as several tens to several hundreds of pounds between the bottom of the well and the formation will exist. This pressure difference would cause the bottom water to flow into the well bore via the tiny clearance between the cement milk and the well wall; furthermore, the water-out zone could not be blocked at all. As mentioned above, in the case of stopping water invasion with the thermosetting resin, the formation water would still break through the joint of the gravels and the well wall, provided the differential pressure at the well bottom exceeds 50 pounds.